Underwire= more support, better fit for me However underwire= occasionally the underwire pokes out of the bra and gives you a death stab in the side boob. When you're at work and you get the underwire boob stab you have to go do bra surgery to be able to make it.
I strongly don't like underwire bras, but some of the sexier ones do have it so for those I'll make the exception. I find it very uncomfortable and not necessarily more supportive. I am a full C cup. Normally I prefer lacy bralettes, I don't really have an issue with breasts sagging so maybe underwire provides the correct lift for these girls and girls bigger than me.
The underwire one is for shirts or dresses that cover up ur shoulder, the other is for shirts and dresses with no straps.
Girls with smaller cup sizes don't need the support, it would just make their boobs look bulky. Girls like me with a larger cup size need the under-wire to A) lift them up a bit obviously B) keep them from jiggling all over the place and C) to take some pressure off our shoulders.